Jet printers (often called "ink jet printers" because they have been used extensively in the printing of alphanumeric characters on paper with a printing ink) are well known. Such printers usually produce a continuous fine stream of droplets when a pressurised supply of a liquid such as ink or a dye is connected to and issues from a small orifice. The individual droplets in the stream are charged as they leave the orifice. They are then deflected with an electrostatic field so that they strike a surface to be printed at a required point or are directed to a collector without reaching that surface.
The droplet production arrangement most widely used in such printers is that described by R. G. Sweet in the specification of his U.S. Pat. No. 3,596,275. In this arrangement, uniform droplets are formed from a liquid jet as it issues from a fine nozzle. Some of these droplets are charged by a charging electrode at the instant the droplet breaks off from the stream from the nozzle, and are subsequently deflected by an electrostatic field to specific recording sites on a surface to be printed. The amplitude of the deflection of a droplet is in proportion to the charge it has acquired from the charging electrode. Droplets which are not charged at the break off time are not deflected by the electrostatic field and are caught, before hitting the surface to be printed, by a collector (usually called a gutter) and are recycled to the liquid supply.
In practice, when operating this type of jet printer, several difficulties are experienced. These difficulties are mainly related to the correct formation of the droplets, the presence of concurrently formed satellite droplets, and the inducing of the charge on the droplets at the instant of formation (failure to correctly charge the droplets results in inaccurate and unreliable printing).
An alternative arrangement for this type of process has been described by R. G. Sweet and R. C. Cumming in the specification of their U.S. Pat. No. 3,373,437. In this arrangement, multiple streams of liquid issue from orifices arranged in a linear array. As these streams break up into droplets, some of the droplets are charged. Those droplets that are charged are electrostatically deflected to a collector or gutter and the uncharged droplets continue to the surface to be printed. When using this type of jet printer, difficulties similar to those outlined above are experienced, and in addition there are difficulties associated with the provision of a large number of jets and the close spacing of their orifices and charging electrodes. Because the single modulating element used to create the droplets does not impart the same stimulation energy to each liquid stream, the streams break up into droplets at different distances from the orifice and precise charging of individual droplets is impossible.
To some extent these difficulties have been obviated in the apparatus disclosed by C. H. Hertz in the specification of his U.S. Pat. No. 3,416,153 and by R. L. Gamblin in his published UK specification No. 2,108,433. Both of these specifications describe arrangements in which an unstimulated liquid jet is used and the natural breakup of the stream into non-uniform, or randomly sized, droplets is allowed to occur.
In the Hertz process, the droplets are dispersed radially from the projected jet axis in the form of a cone by the mutual repulsion of charge induced by a single annular charging electrode placed in close proximity to the stream at the break off point. The dispersed droplets are prevented from reaching the surface to be printed by an annular collector. The absence of a voltage on the charging electrode leaves the droplets uncharged. The uncharged droplets continue undispersed through the central aperture of the collector and strike the printing surface.
A limiting feature of the Hertz process is that single droplet resolution is difficult to achieve and more usually several droplets strike the surface being printed at every point. Furthermore, the process is best suited to very fine jets so that the mutual coulombic repulsion dispersion can take place in a practical distance from the orifice. This feature gives the Hertz process only limited usefulness in printing applications where large volumes of fluid are required (such as the printing of textile fabrics).
In the Gamblin approach, droplets of random size are formed from unstimulated jets. According to Gamblin, the randomly formed droplets from a single jet have a narrower distribution of size and breakoff point variability than had been thought previously, so that jet printers similar in form to that disclosed by Sweet and Cumming in the specification of U.S. Pat. No. 3,373,437 can be made to operate reliably and with acceptable printing accuracy using unstimulated fluid streams. But there is a major difficulty in practising the Gamblin approach, namely the aforementioned problem associated with the large number and close spacing of the jet streams, and the need to closely surround each droplet stream with the required individual charge electrode.
A different approach to droplet production has been described by R. A. Toupin in the specification of his U.S. Pat. No. 3,893,623. Toupin modulates a fine liquid stream to give it an initial varicosity, which grows and results in droplets of different diameter being formed from the stream. The droplets which have a diameter greater than a predetermined value impinge upon a curved surface of a weir and attach themselves to the weir in accordance with the Coanda effect. Smaller droplets clear the weir (and a subsequent baffle) and may be electrostatically deflected before striking a surface to be printed. A closely spaced array of jets can utilise a single weir and baffle.
The Toupin technique avoids the problems associated with the charging of droplets at the instant of their formation, but the selection of printing droplets from the stream is effected near the point of droplet breakoff. Thus the Toupin arrangement, in practice, requires droplets to have a long trajectory before striking the surface to be printed, which is undesirable for accurate printing.
Although jet printers using droplets are the most common form of jet printers, there have been proposals to control a jet of printing liquid by deflecting portions of it. One such proposal is that developed by N. E. Klein and W. H. Stewart and disclosed in the specification of their U.K. patent No. 1,456,458. The technique described in that specification requires that each jet stream in a linear array of liquid streams issuing from an orifice plate may be deflected by a current of air. The currents of air are directed against their respective streams by hollow tubes placed in close proximity to the streams. The deflected streams are caught in a gutter. Each current of air is controlled to be either flowing or absent by an electrically operated pneumatic valve. The jet stream of recording fluid strikes the printing substrate when this valve is in the "off" condition and the current of air does not impinge on and deflect the liquid stream.
This method is inherently reliable in the fluid control domain but has a relatively low frequency response of stream deflection which is limited primarily by the switching speed of the electro-pneumatic valves available and the limitations imposed by the velocity of sound in air. This low frequency response translates to low spatial resolution on the printing surface.
Another stream deflection apparatus--a recorder--is described in the specification of U.S. Pat. No. 1,941,001, granted in 1933 to Clarence W. Hansell. In Hansell's recorder, an unbroken liquid stream is attracted by an electrode to which a high voltage has been applied. The deflected stream of liquid may be intercepted by a baffle placed between the nozzle from which the stream emanates and the printing surface. When this arrangement is used, only that part of the stream which is undeflected reaches the printing surface.
In principle, the Hansell apparatus, should function effectively when a mark/no mark signal recording arrangement is required. It is believed that such apparatus does function well for short periods of time (for example, when recording during a scientific experiment) but problems have been experienced when similar apparatus has been used in a prototype jet printer. The main problem arises because the transition of the stream from one trajectory to another takes a finite time. The leading edge of the baffle intercepts liquid in the transition region between the deflected and undeflected streams. This leads to a build-up of liquid which reduces the selectivity of the baffle (collector), leading to poor resolution and unwanted fouling on the printed surface.